Ni--Al intermetallic compounds, such as Ni.sub.3 Al or the like, have been attracting people's attentions for advanced applications recently due to their many extraordinary properties, such as a high melting point without the transformation of the solid solution commonly observed in nickel-base superalloys, and increasing yield strength with temperature due to thermally activated cross-slip pinning process. In addition, the intrinsic brittleness of polycrystalline Ni.sub.3 Al compound at ambient temperatures has been eliminated by microalloying with boron (B, 0.1 percent by weight). These make it extremely attractive for aviation and structural applications at elevated temperatures. However, the superalloy used in turbine industries has not been replaced by Ni.sub.3 Al. The major problem in manufacturing with melting and casting technique is the strong tendency of Al to oxidize at elevated temperatures, which causes metal-crucible and metal-ceramic interactions during vacuum melting and vacuum investment casting, respectively, as during the regular processing of turbine blades. The increasing yield strength with temperature characteristics also causes the problem of selecting the suitable die material for the post-ingot wrought deformation as a wrought material after ingots have been formed.
Powder metallurgical methods have been alternatively studied by melting raw metals into an alloy and atomizing the alloy into intermetallic powders. It is therefore called pre-alloying powdering method. The thus obtained intermetallic powders adopted as a starting material for powder metallurgical process have the following shortages of suffering from:
1. a required high energy consumption due to an additional process for melting;
2. difficulties on molding to powder compacts due to the hardness of the obtained pre-alloying powders;
3. a high wearing rate caused by them to the mold;
4. a tendency of getting oxidized; and
5. a required high sintering temperature.
There is another method called "mechanical alloying" to finely grind and uniformly mix pure elemental powders by using a high power ball mill. However, the hardening of the ground powder particles makes them not easy to be molded and shaped. In addition, the contamination resulting from the oxidization of the powders during grinding and the degradation of the surface of the balls in the ball mill or of the inner wall of the ball mill are unavoidable. Instead, some of those skilled in the art also set forth the related study by taking pure elemental powders, such as pure Ni and pure Al powders, as a starting material. Three of the representative prior arts, Powder Metal. Int., Vol. 20, No. 3, 25, 1988, J. of Metals, 14, Sep., 1988, and U.S. Pat. No. 4,762,558, all were disclosed by R. M. German et al., report a process, called reactive sintering, executed under a low sintering temperature by taking advantage of the evolved heat and a temporarily formed transient liquid phase during the reaction of the powders. This method is still far from serving as a practical usage, and lacks reports about the mechanical properties of the sintered products to be foundedly supported. Furthermore, a large amount of pores, about 20% of pore density, are formed when the sintering temperature is directly raised to about or above 800.degree. C. Besides, a compound NiAl is possibly formed accordingly to provide a product, being hard and having a low ductility, difficult for further processing. Even at an elevated temperature, the product is still hard. Owing to the low ductility and the high pore density of the sintered product, the product is too hard to process and too brittle to free from cracking so that the cracks of the product have already been resulted before the pore having been able to be healed during processing. It is a common problem people have to face upon manufacturing an intermetallic compound product.
In summary, the shortages of the prior processes include:
1. The product produced thereby has defects in structure;
2. The product produced thereby has poor mechanical properties;
3. One cannot control the temperature distribution in the product during the sintering process so that it is unable to inhibit the formation of the unwanted compounds;
4. The product produced thereby is unsuitable for further hot or cold processes;
5. It is unable to effectively eliminate the pores in the formed product;
6. The product produced thereby is difficult to be molded or shaped; and
This invention is affordable to improve the product density to prevent the product from cracking and capable of solving the aforementioned problems.